Well, in that case you can start by fading oil paints on the metal areas, in order to slightly discolor them and sort of kill the shinyness of the metal particules. You can use whetever colors you fancy but, assuming your base is a silver metal, Ivory Black and Burnt Umber, with the occasional touch of Raw Umber, give some results.
Procedure to follow:
0. Gloss varnish your model
1. Apply tiny dots of oils paints randomly on the surface, making sure that upper areas have more light-colored dots, and lower areas more dark-colored dots.
2. Damp a brush in oil paint thinner (white spirit, turpentine, etc.). Drag it over repeatedly over a surface to wipe out most of the paint dots. Clean you brush in turpentine very repeatedly during this step. You should not wipe all of the paint, but leave a sort of glaze that you alter the color underneath
3. Let dry 24-48 hours
4. Inspect the model: if dots have escaped your attention, repeat the process. If you believe the discoloration is too heavy compared to what you want to achieve, fade the oils a second time.
5. After you like the effect you've achieved, and it's dry for about 48 hours, spray varnish to seal the oil paint layer, and go ahead.